Static Overload in VVC
plus
mode
When the frequency converter is overloaded (the torque limit in par. 4-16
Torque Limit Motor Mode
/par. 4-17
Torque Limit Generator Mode
is reached),
the controls reduces the output frequency to reduce the load.
If the overload is excessive, a current may occur that makes the frequency converter cut out after approx. 5-10 s.
Operation within the torque limit is limited in time (0-60 s) in par. 14-25
Trip Delay at Torque Limit
.
2.13.1 Motor Thermal Protection
This is the way Danfoss is protecting the motor from being overheated. It is an electronic feature that simulates a bimetal relay based on internal
measurements. The characteristic is shown in the following figure:
Illustration 2.11: The X-axis is showing the ratio between I
motor
and I
motor
nominal. The Y- axis is showing the time in seconds before the
ETR cuts off and trips the drive. The curves are showing the characteristic nominal speed at twice the nominal speed and at 0,2x the nominal
speed.
It is clear that at lower speed the ETR cuts of at lower heat due to less cooling of the motor. In that way the motor are protected from being over heated
even at low speed. The ETR feature is calculating the motor temperature based on actual current and speed. The calculated temperature is visible as a
read out parameter in par. 16-18
Motor Thermal
in the frequency converter.
The thermistor cut-out value is > 3 k
Ω
.
Integrate a thermistor (PTC sensor) in the motor for winding protection.
Motor protection can be implemented using a range of techniques: PTC
sensor in motor windings; mechanical thermal switch (Klixon type); or
Electronic Thermal Relay (ETR).
2 Introduction to VLT HVAC Drive
VLT
®
HVAC Drive Design Guide
48
MG.11.B9.02 - VLT
®
is a registered Danfoss trademark
2